相关疑难解决方法(0)

我已经定义了以下模块来帮助我进行FFI功能导出:

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, TypeSynonymInstances #-}
module ExportFFI where

import Foreign
import Foreign.C


class FFI basic ffitype | basic -> ffitype where
    toFFI :: basic -> IO ffitype
    fromFFI :: ffitype -> IO basic
    freeFFI :: ffitype -> IO ()

instance FFI String CString where
    toFFI = newCString
    fromFFI = peekCString
    freeFFI = free

我正在努力实现函数的实例.有人能帮我吗？